Since the advent of the Internet, the popularity of electronic mail or “email” has grown to the point where it is now widely used for both personal and business communication. It is widely recognized that the viability of email is threatened by a problem in the form of the proliferation of unsolicited emails, often mass generated by automated means. Mass emails of this type are known as “Spam” and the orchestrators of Spam as “Spammers”.
The problem has developed to a point where businesses are reluctant to publish their email addresses on their web sites, individuals are reluctant to include their email address in News Group postings, and some people are finding it necessary to periodically abandon their email address and to acquire a new one.
Electronic mail has become a significant means of communication between businesses and individuals alike, and the disruption being inflicted by unsolicited email is imposing a significant cost in both financial and social terms.
Many promising solutions have been applied to the problem. One approach is to install a filtering application upon a user's workstation that searches incoming email messages to determine if they contain any of a blacklist of text strings. If an email does contain a blacklisted text string then that email is diverted from the user's standard mailbox and either deleted or placed in a second mailbox. An example of such a filtering system is described in U.S. Pat. No. 6,023,723 issued Feb. 8, 2000 to McCormick, et al. An earlier filtering system is described in U.S. Pat. No. 5,619,648 to Canale, et al. issued Apr. 8, 1997.
Unfortunately, a number of problems are associated with anti-Spam filtering techniques. For example, anti-Spam filters specifically tend to block messages that contain certain words that may be associated with products that are typically peddled by Spammers. Such words may refer to pornography, financial services and certain pharmaceutical products for example. As a result the filters inadvertently impose a form of censorship on their users by preventing them from participating in email discussions on certain subjects.
Furthermore, anti-Spam filtering systems walk a fine line between incorrectly blocking legitimate messages and failing to block unsolicited, unwanted messages. Most filtering systems allow the user to adjust the filter to err on one side or the other. Nevertheless it remains the case that some legitimate messages will be treated as Spam and some Spam emails treated as legitimate messages and forwarded to the user. Since a percentage of legitimate messages will be incorrectly filtered out as Spam, a user will need to review all of the blocked messages if he or she wishes to be sure that no legitimate messages have been blocked. Unfortunately such a review exposes the user to all of the Spam messages which is the very problem that the filtering software is employed to overcome.
Apart from anti-Spam filters, another approach to reducing Spam is the so-called sender challenge-response verification system. Examples of such systems are disclosed in U.S. Pat. No. 6,112,227 to Heiner issued Aug. 29, 2000 and in U.S. Pat. No. 6,199,102 to Cobb issue Mar. 6, 2001.
The challenge-response approach to addressing the problem of Spam involves checking whether or not the email's sender's address is on a list, i.e a.“white list” of known legitimate correspondents. If the email's sender's address is not on the white list then a challenge is sent to the sender requesting a reply. If the sender does not respond to the challenge then it is assumed that the sender was an automated Spamming application or “Spambot” and the email is not passed on to its intended recipient. Since most Spam is generated by automated Spamming applications, the challenge-response system cuts down on a large portion of unwanted email.
However, one of the disadvantages of using a traditional challenge-response system arises when the user wants to subscribe to an e-zine. It is common for e-zine providers to use automated subscription processes that are not capable of responding to the sender verification system's request for verification, making it necessary for the user to manually white list the e-zine provider. Further, in many cases white-listing the e-zine provider's email address is not possible as many e-zine providers frequently change the sender's address that the sender verification system uses to verify the sender.
Another problem with challenge-response systems is that a user cannot send themselves an email. This is because to do so the user would have to white list their own email address. Doing so would mean that the system could be easily circumvented by Spammers as all that they need do is forge the sender's address to appear that the user is sending themselves an email. In general, white lists, whether used with challenge response or filter systems, may be rendered ineffective by Spammers that collect and use lists of associated email addresses in order to make their Spam appear as if it is coming from a person that is known to the recipient.
Another inhibitor to widespread acceptance of challenge-response anti-Spam systems is the need for the sender to verify that they are human and a reluctance on the part of users to impose this burden on to the legitimate users that send them email.
It is an object of the present invention to provide a method that addresses the problem of unsolicited emails and which is an improvement upon the prior-art approaches discussed above.